Each time a call is set up such an equaliser requires presetting to match it to the transmission channel actually used, and the presetting is provided by transmitting a training signal sequence. The present invention relates more particularly to a synchronous data transmission system of the above-mentioned type which, at least when setting up a link, uses a code that transforms successive groups of n consecutive bits into corresponding successive multivalent symbols, where n is an integer not less than unity, and where the code transforms groups of n consecutive first logic state bits into a null symbol, and a sequence of groups of n consecutive second logic state bits into a sequence of symbols that give rise to a transmitted frequency spectrum which is substantially constituted by a single large-amplitude spectrum line, and which uses an initializing procedure comprising a synchronizing sequence followed by a training sequence, the synchronizing sequence at least ending with a period during which a sequence of groups of n consecutive second logic state bits are transmitted to provide a signal substantially in the form of a single large-amplitude spectrum line, and the training sequence beginning with a least one group of n first logic state bits transmitted in the form of a null symbol.
In particular, use of the present invention makes it possible to detect the beginning of the training sequence for a self-adapting equalizer in a synchronous transmission system which operates in accordance with CCITT recommendation V 37.
Synchronous transmission of data requires, on transmission, the operations of scrambling, coding and possibly modulation and, on reception, the reverse operations: demodulation where necessary, decoding and unscrambling. A filtering operation is also required to correct the distortions generated by the transmission channel.
The coding operation is justified by the works of H. Nyquist who showed that the transmission speed through an ideal low-pass network cannot exceed two data pulses per hertz of pass-band and that it is possible to come close to this theoretical limit by a smoothly rolled-off low-pass filter with a linear phase characteristic. Coding consists in replacing binary data by symbols which may possibly assume more than two values and which are transmitted at a lower rate than the binary data. The symbols are then shaped by filtering to bring the characteristics of the link being used as close as possible to those of a smoothly rolled-off low-pass filter with a linear phase characteristic.
The scrambling operation aims to avoid including large-amplitude spectrum lines in the frequency spectrum.of the transmitted signal due to repetitive sequences in the data to be transmitted since such spectrum lines generate intermodulation noise. Scrambling also facilitates some operations at the receiver end such as clock recovery and filter adjustment to correct the distortions due to the transmission channel. Scrambling takes place before coding and consists in dividing the synchronous binary data to be transmitted by the generation polynomial of a pseudo-random binary sequence. Unscrambling then consists in multiplying the decoded synchronous binary data obtained by the same generation polynomial as is used during the scrambling operation. The scrambler is generally made on the basis of a sequential recursive linear filter and the unscrambler is generally made on the basis of a sequential non-recursive linear filter whose structure is complementary and self-synchronizable. For a detailed description of these structures, reference can advantageously be made to the article by J. E. Savage which appeared on pages 449 to 487 of the February 1967 issue of the BSTJ (Bell System Technical Journal).
Since the filtering operation aims to correct distortions due to the transmission channel, it is carried out by means of a filter called an equalizer which is adjusted so as to obtain, in conjunction with the transmission channel and taking into account the signal shaping after coding, an overall behaviour for the link which is almost the same as that of a smoothly rolled-off low-pass filter with a linear phase characteristic. As a general rule, present equalizers are self-adapting, i.e. they adjust themselves automatically so as to minimize the error which affects the symbols received before decoding. The measurement of this error determines how well the equalizer is adapted, and requires accurate estimation of the transmitted symbols. This estimation can be deduced from the signals from the decoder when the error which affects the symbols applied to it is sufficiently small not to perturb it, i.e. when the equalizer is, in actual fact, near to its optimum adjustment. In contrast, when initially adjusting the coefficients of the equalizer, the error can be deduced only on the basis of prior knowledge of the data transmitted. That is why it is general practice to transmit a succession of binary data called a training sequence during initialization, the composition of this data being known at the receiver end. This sequence is a pseudo-random binary sequence generated at the transmitter end by the scrambler whose input is held at a fixed logic level and, at the receiver end, by the unscrambler which is transformed for this purpose by a set of switches into a scrambler and whose input is held at a fixed logic level. The scrambler and the unscrambler which are then used as two identical and independent generators of pseudo-random binary successions lose their self-synchronization properties. However, synchronization is necessary between the training sequence which comes from the scrambler and is transmitted, received via the transmission channel and made available in coded form at the output of the equalizer, and the coded version of the training sequence generated at the receiver end by the unscrambler, since each shift between these two training sequences results in an identical shift in the coefficients of the equalizer. This shift can be compensated for during self-adaption on real data only if it does not exceed a few symbols. This gives rise to the need at the receiver end to be able to detect the beginning of a training sequence coming from the transmitter end.
One known way of proceeding consists in taking advantage of the fact that the training sequence whose frequency spectrum is composed of a plurality of lines of small and uniform amplitude is preceded in most cases by a synchronization sequence consisting in the transmission of a signal whose frequency spectrum is limited to a few lines of large amplitude to allow clock recovery at the receiver end and also possibly to allow modulation carrier recovery. The instant at which the beginning of the training sequence coming from the transmitter is received can then be deduced from the instant at which the frequency spectrum of the received signal changes and passes from a few large-amplitude lines to a plurality of uniform low-amplitude lines. A band-pass filter is then used whose pass-band is such that the energy at the output of the filter is very low during the synchronization sequence and increases greatly from the beginning of the training sequence. The energy at the output of the filter is measured to make it possible to detect the passage from one sequence to the other. The disadvantage of such a method is that measuring the energy requires integration, and hence causes lack of accuracy on detecting the beginning of the training sequence. This poor accuracy can be up to several tens of symbols with present transmission speeds and is incompatible with the capacity of an equalizer of normal dimensions to self adapt on real data.
Preferred embodiments of the present invention avoid this drawback by taking advantage of the particularities of the code and of the initialization process used when a link is set up, in particular for transmission systems which comply with CCITT recommendation V 37.